A light-emitting diode (LED) is a solid-state semiconductor element, which at least comprises one p-n junction that is formed between a p-type semiconductor layer and an n-type semiconductor layer. When the p-n junction receives certain bias, holes in the p-type semiconductor layer and electrons in the n-type semiconductor layer combine to emit light. The region where light is produced is called an active region. The materials of the p-n junction determine the color of the light emitted inform the active region. For example, LEDs of AlGaInP series emit red light to green light, and LEDs of III-V nitride series emits green light to ultraviolet light.
In general, structures of the active layers comprise single heterostructure (SH), double heterostructure (DH), double-side double heterostructure (DDH), multi-quantum well (MQW), etc. However, such structures still obey the mechanism of the p-n junction. The structure of a commercialized LED production, besides the p-n junction, comprises a growth substrate, a buffer layer, electrodes, a reflection layer, conductive wires, and/or a phosphor, etc.
The light produced in the active layer goes omnidirectionally. However, users usually need the light with specific directions only, so the reflection layer and mirror are adopted to reflect a portion of light. In addition, the difference of the refraction coefficients between LED materials and environment mediums results in that light illuminating on the boundary of the LED undergoes total reflection over a particular incident angle. Generally, it is difficult to prevent each of the reflected light beams from being re-reflected inside the LED.
Referring to FIG. 1A, a prior art LED 100 includes a substrate 110 and epitaxial layers 130. The epitaxial layers 130 include an active layer 131. The active layer 131 emits light omnidirectionally when a bias is applied thereon. A reflection layer 150 is formed between the epitaxial layers 130 and the substrate 110 to reflect the light from the active layer 131.
A first ray R1 emits toward the upper side of the LED 100. When the refraction coefficient of the environment medium is less than that of the LED 100 and the incident angle is larger than the critical angle, the first ray R1 is reflected totally on the boundary of the epitaxial layers 130 toward the inside of the epitaxial layers 130. When the first ray R1 passes through the active layer 131, a portion of the first ray R1 is absorbed by the active layer 131. The other portion of the first ray R1 proceeds toward the reflection layer 150 and is reflected upward to pass through the active layer 131 again. Therefore, the first ray R1 oscillates in the epitaxial layers 130 and passes through the active layer 131 repeatedly, then is gradually absorbed. Under the same mechanism, a second ray R2 emitting to the underside of LED 100 oscillates in the epitaxial layers 130 and passes through the active layer 131 repeatedly, then is absorbed gradually as well.
Referring to FIG. 1B, there is no reflection layer between the substrate 110 of the LED 100 and the epitaxial layers 130, and the substrate 110 is transparent relative to the light emitted from the active layer 131. There is a mirror layer (not shown) or just air beneath the substrate 110. If a third ray R3 reflected form the underside of the substrate 110 illuminates the lateral wall of the substrate 110 with an incident angle ΘI larger than the critical angle ΘC, it will be reflected to inside of the epitaxial layers 130. The active layer 131 absorbs a portion of the third ray R3 back to the epitaxial layers. As described above, the third ray R3 is possible to be totally reflected from the margin of the epitaxial layers 130 to the inside of the epitaxial layers 130, and oscillates in the epitaxial layers 130 as well as passes the active layer 131, then is gradually absorbed. The light absorbed by the active layer 131 definitely reduces the light extraction efficiency of the LED 100 to a certain extent.